I'd recommend OWA for the quick win as listed above. But, do you have RPC over HTTPS setup on your server? If it works for Outlook clients on Windows, Mac Mail setup is very similar and even easier in my opinion.

Try testexchangeconnectivity.com for help in troubleshooting mail client access from off network.

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Also check DNS and be sure you have updated your Autodiscover record. In addition you need to be sure that the DNS entry you use to setup the Mac internally is the same externally, i.e. mail.yourdomain.com...

I find Apple products really need that autodiscover and continuity in the DNS entries to function correctly.

I use Mac Mail to access my Exchange account all the time. Setup is very easy. Like Paul3215 said, just make sure you use your OWA address as the server address. Outlook 2011 setup on a Mac is no different from Outlook setup on a PC.

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I chased this same problem down a few weeks back. The problem was that every time outlook for mac opened, it would grab the local address of the exchange server, so when offsite would no longer connect. DataGuysSD is correct about outlook for mac and autodiscover: it will overwrite the manual settings when it starts. the solution is outlined in this article: